No Sim Error

So I've had my iPhone 6 since launch day and for the past two weeks or so, at least once a day, my phone will revert to "no sim" status and act as if there is no sim installed. Sometimes a reboot helps and sometimes it doesn't.
I took it to a T-Mobile store and the sales woman hooked my phone up to something and installed an app but my phone rebooted and was fine so she sent me on my way.
Has this been happening to anyone else? Am thinking the logical thing to do would be to get a new sim. Does the Apple Store have T-Mobile sims?